1. Field of the Invention
The present invention relates to the technology of electrodepositing a cobalt-chromium film on aluminum alloys, in particular to a process for making a cobalt-chromium magnetic recording film useful in a magnetic recorder by the use of the pretreatment of aluminum alloys either with phosphate or with zincate prior to the electrodepositing.
2. Description of the Prior Art
A magnetic recorder may be used as a precision sensor for controlling and measuring position, speed, and angle. Recently, with the need of a precisely automatic controller of the servomotor, the magnetic recorder appears to be important. For designing a recorder of a magnetic resistance element, there is in need of a hard magnetic drum, and after being magnetized, the magnetic field produced by the magnetic drum is sensed by a MR sensor and is useful to control motors and precision instruments, etc. While making a magnetic drum, it is desired that, after being magnetized, the magnetic resistance sensor can produce a larger voltage swing and clearer signals. In addition, it is further desirable that the deposited film possesses a fair amount of hardness and adherence in respect to the ambient environment where the magnetic recorder is located.
The processes for making a magnetic recording film can be categorized into three groups as follows:
(1) Coating magnetic powders to form a magnetic film: U.S. Pat. No. 4,075,672, for example, discloses a disk of magnetic recording element and a process of producing it by coating technology, by which process magnetic composition powders are coated onto the surface of a substrate. Although the process is convenient, there are shortcomings, such as insufficient delicacy and the lack of vertical anisotropy in the magnetic film. PA1 (2) Depositing a cobalt-chromium alloy on a non-magnetic material by sputtering: U.S. Pat. No. 4,210,946, for example, discloses a magnetic recording medium suitable for use in the vertical recording of signals, which process comprises sputtering a layer of material with low magnetic force and a layer of cobalt-chromium alloy onto a substrate. The advantage of the process is to utilize the content ratios of cobalt-chromium in the deposited film so as to adjust the magnetic characteristics and to increase the magnetization in the vertical direction. The cost of this process is very high because the process requires the use of expensive equipment and needs to operate in vacuum. PA1 (3) Acquiring a magnetic film by use of electrodepositing: U.S. Pat. No. 4,109,287, for example, discloses that commercially pure aluminum (1000 series), 3000 and 5000 series aluminum alloys were opening-activated by an anodic treatment, followed by electrodepositing magnetic films of ferric, cobalt, nickel. The process, however, cannot electrodeposit a cobalt-chromium film having a chromium content of between 1 and 10% by weight and cannot adjust the magnetic characteristics (e.g. coercive force (Hc) and ramanence magnetization (Mr)) by controlling the content ratios of cobalt-chromium as of U.S. Pat. No. 4,210,946. Furthermore, U.S. Pat. No. 4,388,367 discloses activating the surface of a non-conductor such as glass as a substrate by the use of electroless copper plating and electroless nickel-phosphorus plating, followed by electrodepositing to give a cobalt-based magnetic film. Similarly, the process did not involve the surface treatment of aluminum alloys and control different content ratios of cobalt-chromium to electrodeposit the cobalt-chromium magnetic film. In both processes of patents cited hereinabove, although exhibiting the advantages of producing a magnetic film by electrodepositing, they are incapable of changing the cobalt-chromium concentration to adjust the magnetism of the deposited film, making said processes undesirable so that cobalt-chromium magnetic films are generally produced by vacuum sputtering.
Although it has been investigated that a cobalt -chromium alloy film was electrodeposited on copper sheet, based on the reference of Czako-Nagy, M. K. El-Sharif, A. Vertes, C. U. Chisholm, Electrochimica Acta, Vol. 39 No. 6, p 801-805, 1994, the applications of electrodepositing cobalt-chromium films on copper are considerably limited due to the poor mechanical properties and processability of a copper substrate. If the cobalt-chromium were electrodeposited on aluminum alloys, it would be useful because of superior machinability and low density of aluminum alloys.
In order to solve the problems mentioned above, the present invention performs a series of tests of electrodepositing a cobalt-chromium (Co-Cr) magnetic film onto aluminum alloys. Because of aluminum alloys having good mechanical properties and excellent mechanical processability, it was chemically polished and then pretreated either in phosphate or in zincate, and electrodeposited in a plating bath containing chromium and cobalt ions so as to prepare a Co-Cr thin film having different chromium contents of the cobalt-chromium film. After being magnetized, the Co-Cr film can be provided with an excellent function of magnetic recording. Therefore, the process of the present invention can electrodeposit the Co-Cr thin film onto aluminum alloys and produce a cobalt-chromium magnetic recording film, particularly with flexibly adjustable magnetic characteristics, so that the produced Co-Cr film is useful to the magnetic recorder in view of clear signals in a magnetic resistance sensor.